1. Field of the Invention
The invention relates to absorber rods for nuclear reactors with spherical fuel elements which are exhausted after a single passage through the core and more particularly absorber rods inserted directly into the pile in order to affect the prevailing neutron flux in the reactor by absorber material located in an annular gap between two concentrically arranged cylindrical rod elements.
2. Description of the Related Technology
Absorber rods are used in nuclear reactors to control the reactor output, the startup and shutdown processes, to equalize burnup, and to shut down the reactor. For this reason, they contain a neutron absorbing substance, i.e., an absorber material. The absorber material reduces the neutron flux and thus reactivity of the reactor depending on the immersion depth of the rod into the reactor filled with fuel elements based on its neutron capture cross section.
The neutron flux in the reactor attains its maximum flow density as a function of the burnup state of the fuel elements at different heights of the reactor. If the radiation intensity, i.e., the radioactivity of the fuel elements varies, the maximum neutron flux also changes.
In nuclear reactors having piles of spherical fuel elements, in contrast to nuclear reactors with block or rod shaped fuel elements, it is possible to replace fuel elements continuously, without interrupting the operation of the reactor, and thereby to affect the burnup state of the fuel elements in nuclear reactors having piles of spherical fuel element . The reactor may be adjusted so that the fuel elements are used up after a single passage and replaced by new ones. This operational principle is also called the OTTO principle (OTTO=once through then out). In reactors operated by the OTTO principle, the maximum of the neutron flux is located in an intermediate space between the pebble pile and the reactor cover. The absorber rods are inserted through this space into the reactor.
For reactor specific reasons the absorber rods project in their rest position into the reactor space above the pebble pile and are therefore constantly exposed to the reactor atmosphere. In the case of nuclear reactors according to the OTTO principle, the absorber rods in this area are exposed to additional neutron irradiation stresses.
Additional mechanical stresses appear upon rod insertion into the pile as there are no guide installations in nuclear reactors with piles of spherical fuel elements. These additional stresses result from forces against the fuel elements which oppose the insertion of the absorber rod. This resistance of the pile to insertion increases with the depth of the insertion. Absorber rods are only supported in a guide area in armored tubes in a fashion similar to a cantilever beam. Depending on its free length and section modulus the free end immersing into the peeble pile may be deflected from its immersion axis. Accordingly, the absorber rod is exposed to a lateral force producing a bending moment in addition to the force acting in a direction opposing its penetration. It is therefore necessary to take these types of operational mechanical stresses into account in the design of absorber rods for a nuclear reactor They must be correlated with the already present stressing of the rods. Absorber rods are stressed thermally upon their immersion in the reactor in two respects. The radiation heat emitted by the fuel elements leads to a heating of the rod and heat is generated in the absorber material of the absorber rod as the result of neutron absorption. An unacceptable increase in the temperature of the absorber rod due to these heat sources, i.e., an increase in temperature to a value at which the rod would lose its minimum mechanical strength, must be safely excluded. The same is true for the case in which the absorber rod would lose its necessary elasticity and ductility due to neutron embrittlement. Exposure to neutron radiation is, as set forth above, dependent on the layout and the mode of operation of the reactor, i.e., the position of the maximum neutron flux density in the reactor. Mechanical stresses are functions of geometrical parameters, such as the rod cross section, core diameter or core height and thermal stresses are determined by the fuel element inventory.